The growth in telecommunication traffic is driving the development of technology to ever expand the available bandwidth of network communication. In particular, there is a great impetus towards increasing the capacity of optical communication links and reducing the costs of implementing capacity-increasing technologies. One method for increasing the capacity of optical communication links has been to combine optical signals with multiple wavelengths onto the same fiber, commonly referred to as wavelength division multiplexing (WDM). Essential to the development of WDM systems is the development of optical amplifier technology capable of boosting optical signals in a way that is transparent to data rate and format and that can provide effective amplification across a very large bandwidth.
Continuing development of WDM systems including DWDM (dense WDM) systems is increasing the challenges inherent in optical amplifier design. DWDM systems, for example, are capable of carrying hundreds of very closely spaced optical signals (i.e., spacing of less than 25 GHz). Amplifying such DWDM signals is problematic for a number of reasons. For example, if an unstabilized pump laser is used for pumping a Raman optical amplifier used for amplifying DWDM signals, the amplification range of the Raman optical amplifier will be limited by the limited dynamic range of the output power of the unstabilized pump laser. More specifically, if the output power of the unstabilized pump laser is altered by changing the injection current in an attempt to produce a more uniform gain across a given bandwidth, the average temperature of the laser chip is also altered. The change in the average temperature of the laser chip results in a change in the center emission wavelength of the laser and in turn changes the Raman gain of the Raman optical amplifier. As such, the power of an unstabilized pump laser must remain substantially constant, which results in a limited dynamic range.
To avoid the problems associated with using unstabilized pump lasers as optical pumps for Raman optical amplifiers, Fabry Perot (FP) type laser diodes with Fiber Bragg Grating (FBG) wavelength stabilization are typically used as pump sources for Raman amplifiers in high density and high speed optical transmission systems. Such laser diodes provide excellent wavelength stabilization and fixed output wavelengths. The relative intensity noise (RIN) of such stabilized lasers, however, is often insufficient for the stringent requirements of high speed and high density optical systems and applications.